An energy storage and recovery system (ESRS) allows unwanted energy in a machine to be stored, and subsequently to be used to drive, or help to drive, the machine. This improves the overall efficiency of the machine. Examples are the storage of energy that would otherwise be wasted through braking when a vehicle slows down, and the storage of energy returned to a drive mechanism when a heavy component such as a hydraulic arm is lowered. In principle, an energy storage and recovery system can use any form of energy storage device that is capable of receiving energy in the form of motion (usually in the form of rotation) and storing the energy, and is capable subsequently of returning some of the stored energy in the form of motion, again normally in the form of rotation. Normally, a flywheel is used as the energy storage device.
In order to enable an efficient use of the energy storage and recovery system, it is preferable to include, in the transmission between the energy storage device and the energy source/sink (the mechanism from which energy is received and to which energy is delivered), some means for varying the transmission ratio so that the system as a whole can accommodate a large range of speed differences between the energy storage device and the energy source/sink. Additionally, it will normally be desired for the relative speed of movement of the energy storage device and the energy source/sink to change substantially continuously. For example, if the energy storage device is a kinetic energy storage device such as a flywheel, and the energy source/sink is a connection to a vehicle transmission, the transfer of energy will normally imply that the speed of rotation of the flywheel increases as the speed of rotation of the vehicle transmission decreases (storage of braking energy) or vice versa (acceleration using stored energy). This implies that the transmission ratio between the energy storage device and the energy source/sink must change continuously in order to allow energy to be transmitted in the desired manner. Even when the transfer of energy does not require a change of speed at the energy storage device or at the energy source/sink (e.g. the energy storage device is an electric generator/motor and a battery, or the energy source/sink is a lifting device), the energy storage and recovery will often be more efficient if the transmission ratio between the energy storage device and the energy source/sink can be varied. It is known to provide an energy storage and recovery system with a continuously variable transmission such as a belt drive with means to vary the drive radius of a pulley for the belt or alternatively a toroidal continuously variable transmission such as is described in WO 2009/030948.
US 2011/0071000 proposes a system in which a flywheel transmits positive torque (to deliver energy) or negative torque (to store energy) to the wheels of a vehicle through a clutch, and a continuously variable transmission is located in a drive path between the clutch and the vehicle wheels. The system determines the amount of torque to be applied by the flywheel, and this is used to calculate the torque to be transmitted by the clutch. This is used to calculate the clutch pressure to be applied to the clutch. The clutch slips during transmission of torque, and because the torque transmitted by a slipping clutch is determined by the effective friction of the clutch surfaces and the applied clutch force, the torque is independent of the clutch slip speed. The system has a desired clutch slip speed, and determines the desired direction of slip depending on whether the torque to be applied to the wheels is positive or negative (i.e. acceleration force or braking force). The actual clutch slip speed is measured and a slip error is calculated. The continuously variable transmission is adjusted in order to minimise the slip error. Although the clutch slip speed does not affect the clutch torque, the amount of energy lost by frictional heat generation in the clutch does depend on the slip speed. Therefore, the desired clutch slip has a low magnitude.
WO 2011/048102 proposes a system in which energy stored in a flywheel is used to provide torque fill-in during periods of interruption of torque caused by a gearshift event in a vehicle with an automated manual transmission. The flywheel is connected to a variator via an isolating coupling, and the variator is mechanically coupled to the vehicle driveline. In the variator, only speed is controlled which makes it difficult to control torque. Therefore a regulating coupling, including a slipping clutch, is provided between the variator and the vehicle driveline. The variator speed control is used to maintain a consistent but limited slip across the clutch.
In the present application, the term “variable slip transmission” or “VST” is used to refer to a transmission in which a first drive path, including a first clutch, would have a first drive path ratio if the first clutch transmitted drive without slipping, and a second drive path, including a second clutch, would have a second drive path ratio if the second clutch transmitted drive without slipping, wherein the first and second clutches are in parallel and are operable to slip substantially continuously while transmitting drive. Because the clutches slip while transmitting drive, the actual drive path ratio of a drive path, when its clutch is engaged, depends on both (a) the drive path ratio that the drive path would have if the clutch was not slipping and (b) the slip speed (rate of slip) of the clutch. Optionally, there may be further drive paths, which may have further clutches (operable to slip substantially continuously while transmitting drive) in parallel with the first and second clutches. As the rate of slip (slip speed) of a clutch changes, the effective transmission ratio (i.e. the ratio of rotational velocity at one end of the transmission to rotational velocity at the other end of the transmission) also varies. Accordingly, by allowing the slip speed of the engaged clutch to vary, and switching clutches as appropriate, it is possible to provide a transmission with a behaviour similar to that of a continuously variable transmission. Nevertheless, it can still be difficult to transmit torque smoothly between the energy storage device and the energy source/sink. It is desirable to transmit torque smoothly so that the effect of the energy storage and recovery system on the torque applied to the vehicle drivetrain or other energy source/sink does not cause jerks or other rapid changes of torque. An example of a variable slip transmission is known, from WO 2011/080512.